What the FDA labels say about taking them together
Fosphenytoin and phenytoin
Fosphenytoin's FDA label of May 18, 2025 names phenytoin in its boxed warning section; phenytoin's label does not name fosphenytoin itself but warns about its class (Anti-seizure medicines). The fosphenytoin label: “Do not, therefore, make any adjustment in the recommended doses when substituting CEREBYX for phenytoin sodium or vice versa.”
Check it at the source: the fosphenytoin FDA label on DailyMed (effective May 18, 2025, Pfizer Laboratories Div Pfizer Inc); the phenytoin FDA label on DailyMed (effective October 15, 2024, Mylan Pharmaceuticals Inc.).
What does the fosphenytoin label say about phenytoin?
Fosphenytoin label, boxed warningCARDIOVASCULAR RISK ASSOCIATED WITH RAPID INFUSION RATES The rate of intravenous CEREBYX administration should not exceed 150 mg phenytoin sodium equivalents (PE) per minute in adults and 2 mg PE/kg/min (or 150 mg PE/min, whichever is slower) in pediatric patients because of the risk of severe hypotension and cardiac arrhythmias.
Fosphenytoin label, contraindicationsCONTRAINDICATIONS CEREBYX is contraindicated in patients with: A history of hypersensitivity to CEREBYX or its inactive ingredients, or to phenytoin or other hydantoins [see Warnings and Precautions (5.6) ].
Fosphenytoin label, drug interactionsThe most significant drug interactions following administration of CEREBYX are expected to occur with drugs that interact with phenytoin.
Fosphenytoin label, warnings and precautionsDo not, therefore, make any adjustment in the recommended doses when substituting CEREBYX for phenytoin sodium or vice versa.
Source: FDA drug label (CEREBYX), NDA020450 (Parke Davis), as published by Pfizer Laboratories Div Pfizer Inc, effective May 18, 2025 (SPL set de5a6711-4abb-4c3d-a18b-266b094ace11), via openFDA; the full label on DailyMed
What does the phenytoin label say about fosphenytoin?
Phenytoin's label does not name fosphenytoin itself; it warns about its class (Anti-seizure medicines):
Phenytoin label, drug interactionsDrugs That Affect Phenytoin Concentrations Interacting Agent Examples Drugs that may increase phenytoin serum levels Antiepileptic drugs Ethosuximide, felbamate, oxcarbazepine, methsuximide, topiramate Azoles Fluconazole, ketoconazole, itraconazole, miconazole, voriconazole Antineoplastic agents Capecitabine, fluorouracil Antidepressants Fluoxetine, fluvoxamine, sertraline Gastric acid reducing agents H 2 antagonists (cimetidine), omeprazole Sulfonamides Sulfamethizole, sulfaphenazole, sulfadiazine …
Phenytoin label, drug interactionsPhenytoin and antacids should not be taken at the same time of day Antineoplastic agents usually in combination Bleomycin, carboplatin, cisplatin, doxorubicin, methotrexate Antiviral agents Fosamprenavir, nelfinavir, ritonavir Antiepileptic drugs Carbamazepine, vigabatrin Other Chronic alcohol abuse, diazepam, diazoxide, folic acid, reserpine, rifampin, St. John’s wort The induction potency of St. John’s wort may vary widely based on preparation. …
Source: FDA drug label (Phenytek), Mylan Pharmaceuticals Inc., effective October 15, 2024 (SPL set 0f94c791-8b07-4b41-927e-c9f28bec5f60), via openFDA; the full label on DailyMed
These are the labels' own sentences. PlainMeds does not grade interactions; a pharmacist or prescriber can say what they mean for one person's medicines.
Both medicines on the record
| Fosphenytoin | Phenytoin | |
|---|---|---|
| Labeled for | Seizures and epilepsy | Seizures and epilepsy |
| Boxed warning | Yes | No |
| Pharmacy cost (NADAC) | n/a | $0.145 per capsule 100 mg capsule |
| Shortage now | No current entry | No current entry |
Source: FDA NDC Directory, CMS NADAC of October 7, 2026 and the FDA Drug Shortage Database (FDA files October 8, 2026)
What next?
- Check more medicines together
- up to five at once